JP2008274323A - Hot-rolled steel sheet excellent in surface quality and ductile crack propagation characteristics and method for producing the same - Google Patents

Abstract

A method for producing a hot-rolled steel sheet having excellent surface quality and excellent ductile crack propagation characteristics is provided.
A steel material having a composition in which C, 0.02 to 0.08%, Nb: 0.03 to 0.10%, Ti: 0.005 to 0.05%, and Si, Mn, P, S, Al, and N are adjusted to appropriate amounts. In performing the rough rolling step, the finish rolling step, and the winding step in sequence, the surface layer portion is set to 50 ° C./s or more after the rough rolling step and before the finishing rolling step and / or during the finishing rolling step. Accelerated cooling that rapidly cools until reaching a temperature of 930 ° C or more exceeding the Ar ₃ transformation point at the cooling rate, or accelerated cooling that quenches the surface layer portion at a temperature of 50 ° C / s or more until it reaches the temperature of the Ar ₃ transformation point or less. Thereafter, the accelerated cooling is stopped, and then finish rolling is performed. In addition, when accelerated cooling which quenches rapidly until it reaches the temperature below the Ar ₃ transformation point, it is preferable that finish rolling limits the rolling reduction per pass to (1.1 × uniform elongation)% or less. Thereby, it can be set as the high tension hot-rolled steel plate excellent in surface quality, and excellent in toughness, especially ductile crack propagation characteristics.
[Selection figure] None

Description

  The present invention relates to a method for producing a high-tensile hot-rolled steel sheet that is suitable for use in an electric-welded steel pipe and a spiral steel pipe for line pipes, and particularly relates to prevention of surface defects and improvement of ductile crack propagation characteristics. In addition, a steel plate includes a steel plate and a steel strip.

  In recent years, oil and natural gas mining and pipeline construction have been actively carried out in extremely cold regions such as the North Sea, Canada and Alaska due to soaring crude oil since the oil crisis and the demand for diversified energy supply sources. It has come to be. Furthermore, in the pipeline, in order to improve the transportation efficiency of natural gas and oil, there is a tendency to perform high-pressure operation with a large diameter. In order to withstand the high-pressure operation of the pipeline, the transport pipe (line pipe) needs to be a thick-walled steel pipe, and a UOE steel pipe made of a thick steel plate has been used. However, recently, in line with the strong demand for further reduction in pipeline construction costs, there is a strong demand for reducing the material cost of steel pipes. Instead of UOE steel pipes made of thick steel plates as transport pipes, productivity is higher. High-strength ERW steel pipes or spiral steel pipes made from inexpensive, coil-shaped hot-rolled steel sheets (hot-rolled steel strips) have come to be used.

  These high-strength steel pipes are required to maintain excellent low-temperature toughness at the same time from the viewpoint of preventing line pipe breakage. In order to produce steel pipes that have both strength and toughness, steel sheets that are steel pipe materials are produced by transformation strengthening using accelerated cooling after hot rolling and precipitation of alloy elements such as Nb, V, and Ti. Increased strength by precipitation strengthening and the like using slag and refinement of structure using controlled rolling and the like have been achieved. More recently, steel pipes for extremely cold regions may be required to have fracture toughness, particularly excellent CTOD characteristics, particularly excellent DWTT characteristics, from the viewpoint of preventing pipeline burst fracture. Many.

In response to such demands, for example, Patent Document 1 includes C: 0.05 to 0.12%, Ca: 0.0020 to 0.0060%, and includes a continuous cast slab containing Si, Mn, Al, P, and S with appropriate amounts adjusted. In addition, a reduction of 10 to 50% is performed at 950 ° C. or lower, and the cooling is continued until the surface cooling rate is 2 ° C./s or higher and the surface temperature is Ar ₃ or lower. Roll at 50% or more in the crystal region, finish rolling in the range of 720 to 820 ° C, subsequently cool at an average cooling rate of 5 to 30 ° C / s, and then wind in the range of 400 to 600 ° C A method of manufacturing a hot coil for high toughness sour steel pipe is described. According to the technique described in Patent Document 1, a hot coil excellent in both HIC resistance and low temperature toughness can be manufactured, and a line pipe can be manufactured in a cold region.

In Patent Document 2, a steel slab containing C: 0.01 to 0.20% and containing appropriate amounts of Si, Mn, Al, and N is heated to an Ac ₃ transformation point or higher and 1250 ° C. or lower, and a temperature of 900 ° C. or higher. After rough rolling at a cumulative rolling reduction of 10 to 80%, accelerated cooling at 2 to 40 ° C./s is performed at (Ar ₃ transformation point + 50 ° C.) to (Ar ₃ transformation point −50 ° C.) at the cooling rate. ), And after completion of accelerated cooling, finish rolling with a cumulative rolling reduction of 30 to 90% is completed at 650 ° C or higher, and after finishing rolling is further accelerated to 200 to 450 ° C at a cooling rate of 5 to 40 ° C / s. A method for producing a low-yield ratio high-tensile steel material excellent in low-temperature toughness to be cooled is described. According to the technique described in Patent Document 2, a hot-rolled steel sheet having both a low yield ratio and excellent low-temperature toughness can be produced without requiring a complicated heat treatment step.

Patent Document 3 includes C: 0.01 to 0.10%, Nb: 0.01 to 0.1%, includes appropriate amounts of Si, Mn, P, S, and N, and satisfies Mn / Si: 5 to 8. The first rolling reduction performed at 1100 ° C or higher is 15 to 30%, the total rolling reduction at 1000 ° C or higher is 60% or higher, and the final rolling rolling reduction is 15 to 30%. Rolling is performed, the steel sheet surface layer part is cooled to Ar ₃ points or less at a cooling rate of 5 ° C./s or more, and the temperature of the surface layer part is (Ar ₃ −40 ° C.) to (Ar ₃ + 40 ° C.) by reheating or forced heating. ), Then finish rolling was completed under the condition of a total reduction ratio of 950 ° C or less: 60% or more, then cooling was started within 2 s, and 600 ° C at a rate of 10 ° C / s or more. A method for producing a hot rolled steel sheet for a high-strength ERW steel pipe excellent in low temperature toughness and weldability, which is cooled to below ℃ and wound up in the range of 600 to 350 ℃ is described. According to the technique described in Patent Document 3, a high-strength ERW steel pipe excellent in low-temperature toughness and weldability can be manufactured without adding an expensive alloy element and without heat treatment. .
Japanese Unexamined Patent Publication No. 7-268467 JP-A-10-306316 Japanese Patent Laid-Open No. 2001-207220

  However, in the hot-rolled steel sheet manufactured by the technique described in Patent Document 1, the improvement of the HIC resistance is remarkable, but the improvement of the DWTT characteristic and the CTOD characteristic is not remarkable, and surface cracks may occur. There was a problem left. Furthermore, in the technique described in Patent Document 1, the amount of unrecrystallized zone reduction has to be extremely large, an excessive load is applied to the rolling mill, and it is difficult to manufacture a thick plate. There was also a problem. Moreover, in the hot-rolled steel plate manufactured by the technique described in Patent Literature 2 and Patent Literature 3, there is a problem that surface cracks frequently occur.

  The present invention solves such problems of the prior art, and does not generate surface defects such as surface cracks even when subjected to low temperature and large rolling, and has excellent surface quality, and also has excellent low temperature toughness, particularly ductile crack propagation characteristics. It aims at providing the manufacturing method of the high-tensile-strength hot-rolled steel plate which becomes possible to manufacture a steel plate. The “steel plate” here means a steel plate having a tensile strength TS of 490 MPa or more. Further, “excellent surface quality” refers to a case where a surface crack with a depth of 100 μm or more does not occur in a product (steel plate). Further, “excellent in ductile crack propagation characteristics” means a case where the critical opening displacement amount δc at −10 ° C. is 0.25 mm or more in the CTOD test.

In order to achieve the above-described problems, the present inventors have conducted extensive research on various factors affecting toughness and surface quality. As a result, the present inventors have found that surface defects such as surface cracks are directed to low temperature rolling in order to ensure high toughness, resulting in reduced ductility due to overcooling of the surface layer part, or excessive reduction to the surface layer part. To find out that the main cause of cracks in intergranular ferrite. However, according to the study by the present inventors, in order to obtain a high toughness hot-rolled steel sheet, after the temperature of the material to be rolled is cooled to a temperature range effective for toughening, finish rolling is applied to a predetermined range of reduction. It has been found that it is essential to perform the above-mentioned, and for that purpose, it is essential to perform accelerated cooling before or during finish rolling, and to apply an effective rolling rate of a predetermined value or more in subsequent finish rolling. In order to achieve both high toughness and excellent surface quality, the accelerated cooling is adjusted so that the surface layer does not fall below the Ar ₃ transformation point at which the ductility is lowered, or so that it is below the Ar ₃ transformation point. When cooling to a very low temperature, we know that it is better to perform rolling so that the rolling reduction per pass in the subsequent finish rolling is less than the value related to the uniform elongation (uniform elongation) value at high temperatures. did. And it discovered that such a process could be achieved by actively utilizing existing cooling means before finish rolling and cooling means in the finish rolling mill.

The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.02 to 0.08%, Si: 0.5% or less, Mn: 0.8 to 1.8%, P: 0.025% or less, S: 0.005% or less, Al: 0.005 to 0.10%, N: 0.005% In the following, a rough rolling step in which Nb: 0.03 to 0.10%, Ti: 0.005 to 0.05%, and a steel material having a composition composed of the remaining Fe and inevitable impurities is subjected to rough rolling to form a sheet bar, In the method of manufacturing a hot-rolled steel sheet that sequentially performs a finish-rolling step for performing hot rolling and making a hot-rolled plate, and a winding step for winding the hot-rolled plate, after the rough rolling step, before the finish rolling step, The sheet bar is subjected to accelerated cooling in which the surface layer is rapidly cooled at a cooling rate of 50 ° C./s or more until reaching a temperature exceeding the Ar ₃ transformation point and reaching 930 ° C. or less, then the accelerated cooling is stopped, and then finish rolling A method for producing a hot-rolled steel sheet having excellent surface quality and ductile crack propagation characteristics, characterized by performing a process.

(2) By mass%, C: 0.02 to 0.08%, Si: 0.5% or less, Mn: 0.8 to 1.8%, P: 0.025% or less, S: 0.005% or less, Al: 0.005 to 0.10%, N: 0.005% In the following, a rough rolling step in which Nb: 0.03 to 0.10%, Ti: 0.005 to 0.05%, and a steel material having a composition composed of the remaining Fe and inevitable impurities is subjected to rough rolling to form a sheet bar, In the method of manufacturing a hot-rolled steel sheet that sequentially performs a finish-rolling process for performing hot rolling and making a hot-rolled sheet, and a winding process for winding the hot-rolled sheet, at least once in the finish rolling process, between rolling passes, The hot-rolled sheet in the middle of finish rolling is subjected to accelerated cooling in which the surface layer portion is rapidly cooled at a cooling rate of 50 ° C./s or more until it reaches a temperature exceeding the Ar ₃ transformation point and 930 ° C. or less, and then the accelerated cooling is stopped. Furthermore, it is excellent in surface quality and ductile crack propagation characteristics, characterized in that it is finish-rolled to form a hot-rolled sheet of a predetermined size and shape. Method for manufacturing a hot-rolled steel sheet that.

(3) By mass%, C: 0.02 to 0.08%, Si: 0.5% or less, Mn: 0.8 to 1.8%, P: 0.025% or less, S: 0.005% or less, Al: 0.005 to 0.10%, N: 0.005% In the following, a rough rolling step in which Nb: 0.03 to 0.10%, Ti: 0.005 to 0.05%, and a steel material having a composition composed of the remaining Fe and inevitable impurities is subjected to rough rolling to form a sheet bar, In the method of manufacturing a hot-rolled steel sheet that sequentially performs a finish-rolling step for performing hot rolling and making a hot-rolled plate, and a winding step for winding the hot-rolled plate, after the rough rolling step, before the finish rolling step, The sheet bar is subjected to accelerated cooling in which the surface layer portion is rapidly cooled at a cooling rate of 50 ° C./s or more until reaching a temperature of more than 930 ° C. above the Ar ₃ transformation point, and then the accelerated cooling is stopped, and then the finish rolling In addition, at least once in the finish rolling step, between the rolling passes, on the hot rolled sheet in the middle of finish rolling, After the part is subjected to accelerated cooling by quenching to reach Ar ₃ transformation point exceeding 930 ° C. or less of the temperature cooling rate higher than 50 ° C. / s, stop the pressurized-speed cooling, the heat of the desired size and shape further subjected to finish rolling A method for producing a hot-rolled steel sheet, which is excellent in surface quality and ductile crack propagation characteristics, characterized by being a rolled sheet.

(4) In any one of (1) to (3), the accelerated cooling is performed by rapidly cooling the surface layer portion at a cooling rate of 50 ° C./s or more until reaching a temperature not higher than the Ar ₃ transformation point. Finish rolling in the process, the rolling reduction per pass is (1.1 × uniform elongation)% or less (here, uniform elongation: after heating to 950 ° C, cooling to the Ar ₃ transformation point or less, and then to 950 ° C again. A method for producing a hot-rolled steel sheet, characterized by having a uniform elongation value (%) in the obtained stress-strain curve when subjected to a high-temperature tensile test by heating.

(5) In any one of (1) to (4), in addition to the above composition, Cu: 0.005-0.5%, Ni: 0.005-0.5%, Cr: 0.005-0.5%, Mo: 0.005 A method for producing a hot-rolled steel sheet, comprising: one or two or more selected from ˜0.3% and V: 0.005 to 0.3%.
(6) In any one of (1) to (5), the winding temperature of the hot-rolled sheet in the winding step is set to 350 to 700 ° C., and the cooling rate after winding is set to 5 to 20 at the coil central portion. The manufacturing method of the hot-rolled steel plate characterized by setting it as (degreeC / h).

  (7) By mass%, C: 0.02 to 0.08%, Si: 0.5% or less, Mn: 0.8 to 1.8%, P: 0.025% or less, S: 0.005% or less, Al: 0.005 to 0.10%, N: 0.005% In the following, Nb: 0.03 to 0.10%, Ti: 0.005 to 0.05%, the composition composed of the balance Fe and inevitable impurities, and the structure composed of a single phase of bainitic ferrite, with a tensile strength TS of 490 MPa or more A hot-rolled steel sheet having excellent surface quality and ductile crack propagation characteristics.

  (8) In (7), in addition to the above composition, Cu: 0.005-0.5%, Ni: 0.005-0.5%, Cr: 0.005-0.5%, Mo: 0.005-0.3%, V: 0.005 A hot-rolled steel sheet characterized by having a composition containing one or more selected from ˜0.3%.

  According to the present invention, a high-tensile hot-rolled steel sheet having no surface defects such as surface cracks, excellent surface quality, and low-temperature toughness, particularly excellent ductile crack propagation characteristics, can be easily and highly productively manufactured. Has an exceptional effect. In addition, according to the present invention, the material cost of the steel pipe can be reduced, and therefore, there is an effect that the construction cost of the pipeline can be further reduced.

First, the reasons for limiting the composition of the steel material used in the present invention will be described. Note that the mass% is simply indicated as%, unless otherwise specified.
C: 0.02 to 0.08%,
C is an element having an action of increasing the strength of steel, and in the present invention, it is necessary to contain 0.02% or more in order to ensure a desired high strength. On the other hand, an excessive content exceeding 0.08% increases the structural fraction of the second phase such as pearlite, and lowers the base metal toughness and the weld heat affected zone toughness. For this reason, C was limited to the range of 0.02 to 0.08%. In addition, Preferably it is 0.02 to 0.05%.

Si: 0.5% or less
Si increases the strength of steel through solid solution strengthening and hardenability improvement, but at the same time has the effect of lowering toughness. Reduces the toughness of the sewn weld. For this reason, in the present invention, it is desirable to reduce Si as much as possible, but up to 0.5% is acceptable, so Si is limited to 0.5% or less. In addition, Preferably it is 0.3% or less.

Mn: 0.8-1.8%
Mn has the effect of improving hardenability, and increases the strength of the steel sheet through the improvement of hardenability. Further, Mn forms MnS and fixes S, thereby preventing segregation of S grain boundaries and suppressing slab (steel material) cracking. In order to acquire such an effect, 0.8% or more needs to be contained. On the other hand, a content exceeding 1.8% promotes segregation and increases the occurrence of separation. In order to eliminate this segregation, it is necessary to heat to a temperature exceeding 1300 ° C., and it is not practical to carry out such a heat treatment on an industrial scale. For this reason, Mn was limited to the range of 0.8 to 1.8%. In addition, Preferably it is 0.9 to 1.7%.

P: 0.025% or less P is inevitably contained as an impurity in steel, but has an effect of increasing the strength of steel. However, when it exceeds 0.025% and it contains excessively, weldability will fall. For this reason, P was limited to 0.025% or less. In addition, Preferably it is 0.015% or less.
S: 0.005% or less S is inevitably contained as an impurity in steel like P, but if it exceeds 0.005% and excessively contained, it causes slab cracking and coarse hot MnS in hot-rolled steel sheets. Forming and causing a reduction in ductility. For this reason, S was limited to 0.005% or less. In addition, Preferably it is 0.003% or less.

Al: 0.005-0.10%
Al is an element that acts as a deoxidizer, and in order to obtain such an effect, it is desirable to contain 0.005% or more. On the other hand, the content exceeding 0.10% significantly impairs the cleanliness of the welded part during ERW welding. For these reasons, Al is limited to 0.005 to 0.10%. In addition, Preferably it is 0.005-0.08%.

N: 0.005% or less N is an element inevitably contained in steel, but excessive inclusion frequently causes cracks during slab casting. For this reason, N was limited to 0.005% or less. In addition, Preferably it is 0.003% or less.
Nb: 0.03-0.10%
Nb is an element that has the effect of suppressing the coarsening and recrystallization of austenite grains, enabling the austenite non-recrystallization temperature range rolling in hot finish rolling, and by precipitating finely as carbonitride, It has the effect | action which makes a hot-rolled steel plate high intensity | strength with little content, without impairing property. In order to obtain such an effect, the content of 0.03% or more is required. On the other hand, an excessive content exceeding 0.10% may cause an increase in rolling load during hot finish rolling, which may make hot rolling difficult. For this reason, Nb was limited to the range of 0.03-0.10%. In addition, Preferably it is 0.03-0.07%.

Ti: 0.005-0.05%
Ti has the effect of forming nitrides and fixing N to prevent cracking of the slab (steel material), and also makes the steel sheet high in strength by being finely precipitated as carbides. Such an effect becomes remarkable when the content is 0.005% or more, but when the content exceeds 0.05%, the yield point is remarkably increased by precipitation strengthening. For this reason, Ti was limited to 0.005 to 0.05%. In addition, Preferably it is 0.005-0.03%.

The above components are basic compositions. In addition to this basic composition, Cu: 0.005-0.5%, Ni: 0.005-0.5%, Cr: 0.005-0.5%, Mo: 0.005-0.3%, V: It is good also as a composition containing 1 type, or 2 or more types chosen from 0.005-0.3%.
Cu, Ni, Cr, Mo, and V are all elements that improve the hardenability and increase the strength of the steel sheet, and can be selected from one or more as required.

Cu is an element that has the effect of improving the hardenability and increasing the strength of the steel sheet by solid solution strengthening or precipitation strengthening. In order to acquire such an effect, it is desirable to contain 0.005% or more, but inclusion exceeding 0.5% reduces hot workability. For this reason, it is preferable to limit Cu to 0.005-0.5%.
Ni is an element that has the effect of improving hardenability, increasing the strength of the steel sheet, and improving toughness. In order to acquire such an effect, it is desirable to contain 0.005% or more, but even if it contains more than 0.5%, the effect is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous. For this reason, Ni is preferably limited to 0.005 to 0.5%.

Cr is an element that has the effect of improving hardenability and increasing the strength of the steel sheet. Such an effect becomes remarkable when the content is 0.005% or more. On the other hand, an excessive content exceeding 0.5% tends to cause frequent welding defects during ERW welding. For this reason, it is preferable to limit Cr to 0.005 to 0.5%. In addition, More preferably, it is 0.15-0.3%.
Mo is an element that has an effect of improving hardenability and forming carbides to increase the strength of the steel sheet. Such an effect becomes remarkable when the content is 0.005% or more. On the other hand, a large content exceeding 0.3% reduces weldability. For this reason, it is preferable to limit Mo to 0.005 to 0.3%. More preferably, it is 0.1 to 0.3%.

V is an element that has an effect of improving hardenability and forming carbonitride to increase the strength of the steel sheet. Such an effect becomes remarkable when the content is 0.005% or more. On the other hand, an excessive content exceeding 0.3% deteriorates weldability. For this reason, V is preferably 0.005 to 0.3%. In addition, More preferably, it is 0.005-0.15%.
The balance other than the components described above consists of Fe and inevitable impurities.

  The steel material having the above composition is subjected to rough rolling to be a sheet bar, to a rolling process, to finish rolling the sheet bar to be a hot rolled sheet, and to wind the hot rolled sheet Are applied sequentially. In addition, the manufacturing method of a steel raw material does not need to be specifically limited. The molten steel having the above composition can be melted by a normal melting method such as a converter and used as a steel material by a normal casting method such as a continuous casting method or an ingot-bundling method.

  The steel material having the above composition is heated and first subjected to a rough rolling process. Although heating temperature is not specifically limited, It is preferable to set it as the temperature of the range of 1000-1250 degreeC. When the heating temperature is less than 1000 ° C., the deformation resistance is high and the load on the rolling mill becomes excessive. On the other hand, when the temperature is higher than 1250 ° C., the crystal grains become too coarse and the toughness of the hot-rolled sheet decreases. In addition, scale loss increases and yield decreases. In the rough rolling process, it is sufficient that the sheet bar has a predetermined size, and the rough rolling conditions are not particularly limited.

  The sheet bar obtained through the rough rolling process is subjected to a finish rolling process, but it is preferable to perform accelerated cooling before the finish rolling process. The accelerated cooling is performed to cool the sheet bar and the like, cool to a temperature range effective for high toughness, and then effectively improve toughness by finish rolling. By performing accelerated cooling, the region in the plate thickness direction cooled to a temperature range effective for toughening can be expanded, and the degree of improvement in toughness by finish rolling can be increased. The accelerated cooling before the finish rolling process can be easily performed by a cooling means such as an existing FSB, roll heat removal, strip coolant, or the like.

The accelerated cooling is preferably cooling in which the surface layer portion of the sheet bar is rapidly cooled at a cooling rate of 50 ° C./s or more until reaching a temperature of 930 ° C. or more exceeding the Ar ₃ transformation point. Thereby, an effective rolling rate can be increased. Here, “surface layer temperature” is a value measured by a radiation thermometer. When the cooling rate of accelerated cooling is less than 50 ° C./s, it takes time to cool to a temperature range effective for toughening, and productivity is lowered.

When the cooling stop temperature of accelerated cooling exceeds 930 ° C., the range of cooling to a temperature range effective for high toughness is narrow, and the allowance for improving toughness is small. On the other hand, when the cooling stop temperature for accelerated cooling is below the Ar ₃ transformation point, the risk of cracking in the surface layer portion increases depending on the subsequent finish rolling conditions.
Further, instead of the above-described accelerated cooling, it may be accelerated cooling in which the surface layer portion is rapidly cooled at a cooling rate of 50 ° C./s or higher until reaching a temperature not higher than the Ar ₃ transformation point. Thereby, it becomes easy to make the temperature up to 930 ° C. or less, which is a temperature range effective for toughening, up to the vicinity of the center portion of the seat bar. However, in this case, the finish rolling conditions after accelerated cooling must be within a specific range. If the cooling stop temperature of accelerated cooling is below the Ar ₃ transformation point and less than 350 ° C., a problem occurs in the winding process.

The sheet bar subjected to accelerated cooling is then subjected to a finish rolling process. In this finish rolling step, a plurality of rolling mills are arranged in series and continuously rolled.
As described above, accelerated cooling may be performed during the finish rolling process instead of after the rough rolling process and before the finishing rolling process, and after the rough rolling process and before the finishing rolling process. The rolling process may be performed together.

In the case where accelerated cooling is performed during the finish rolling process, it is preferably applied to the hot-rolled sheet in the middle of finish rolling at least once between the rolling mills arranged in series. Accelerated cooling during the finish rolling process can be performed by using a coolant in the finish rolling mill. In addition, the accelerated cooling during the finish rolling process is similar to the accelerated cooling before the finish rolling process, and the surface layer portion of the hot-rolled sheet in the middle of rolling has an Ar ₃ transformation point exceeding 930 ° C. or less at a cooling rate of 50 ° C./s or more. It is preferable that the cooling is rapid cooling to a temperature, or rapid cooling to a temperature not higher than the Ar ₃ transformation point, preferably 350 ° C. or higher.

The hot-rolled sheet that has been subjected to accelerated cooling is subjected to a predetermined reduction in the finish rolling process to obtain a hot-rolled sheet having a predetermined size. In the present invention, the effective rolling reduction in finish rolling is preferably 20% or more from the viewpoint of increasing toughness. The effective rolling reduction means the total rolling amount in a temperature range of 930 ° C. or lower, which is a temperature range effective for high toughening.
When the stop temperature of accelerated cooling exceeds the Ar ₃ transformation point, the rolling conditions of finish rolling in the finish rolling process to be performed thereafter are not particularly limited, but the reduction rate per pass is 15 to 50%. Rolling is preferable. If the rolling reduction per pass is less than 15%, the desired high toughness cannot be expected, and the possibility of warping increases. On the other hand, if the rolling reduction per pass exceeds 50%, surface cracks occur during rolling, and an excessive load is applied to the rolling mill, which is not preferable. More preferably, it is 15 to 30%.

On the other hand, when the accelerated cooling stop temperature is lower than the Ar ₃ transformation point, the structure of the surface layer portion becomes two phases of ferrite and austenite, and in order to avoid the occurrence of surface defects, the present invention stops accelerated cooling. It is preferable to limit the rolling reduction per pass in the subsequent finish rolling to (1.1 × uniform elongation)% or less. “Uniform elongation” as used herein refers to the stress-strain obtained when heating to 950 ° C., cooling to below the Ar ₃ transformation point, and then reheating to 950 ° C. to conduct a high-temperature tensile test. The uniform elongation value (%) in the line. If the rolling reduction per pass exceeds (1.1 × uniform elongation)%, the strain concentrates on the precipitated ferrite, and it is easy to induce cracks in the surface layer.

  The hot-rolled sheet obtained through the finish rolling process is then wound into a coil in the winding process. In the winding process in the present invention, the winding temperature is preferably 350 to 700 ° C. After finishing rolling, it is preferable to cool the hot-rolled sheet to a coiling temperature, preferably at a cooling rate of 10 ° C./s or more. When the coiling temperature is less than 350 ° C., the temperature variation at each position of the steel sheet increases, resulting in material variation and shape variation, and further, it may not be possible to wind depending on the coiler capacity. On the other hand, when the coiling temperature exceeds 700 ° C., the crystal grains become coarse and the toughness decreases. Therefore, the winding temperature is preferably 350 to 700 ° C. Moreover, after winding up in coil shape, it is preferable to cool to room temperature at 5-20 degreeC / h by the cooling rate of the coil center part. If the cooling rate after coiling is less than 5 ° C./h, the crystal grains become coarse and the toughness decreases. On the other hand, when it exceeds 20 ° C./h, it is impossible to cool with a normal winding equipment, and a new capital investment is required. In addition, the temperature difference between the coil central portion and the outer periphery and the inner periphery increases, which tends to cause material nonuniformity.

In addition, since surface cracks are likely to occur during the production of a thick steel plate having a large temperature difference between the surface layer portion and the plate thickness center portion, the effect of the present invention becomes more prominent as the thick steel plate. Here, “thick” means WT17.5 mm or more, particularly WT19,1 mm or more.
A hot-rolled steel sheet obtained under the above-described manufacturing conditions has the above-described composition and a structure composed of a bainitic ferrite single phase, a tensile strength TS of 490 MPa or more, and a depth of 100 μm or more. This steel sheet has no defects (surface cracks) and excellent surface quality, and has excellent ductile crack propagation characteristics with a critical opening displacement amount δc of −0.25 ° C. or more at −10 ° C. in the CTOD test. Here, “bainitic ferrite” refers to a bainite phase that has a high dislocation density in crystal grains and is transformed at a low temperature and hardly shows cementite precipitation between laths.

  A slab (steel material) having the composition shown in Table 1 (wall thickness: 215 mm) is heated to the temperature shown in Table 2, and subjected to a rough rolling process, a finish rolling process, and a winding process under the conditions shown in Table 2. A hot-rolled sheet having a thickness shown in Table 2 was used. In addition, accelerated cooling was performed under the conditions shown in Table 2 after the rough rolling process and before the finish rolling process and / or between passes during the finish rolling process. In the winding process after the finish rolling process, the steel sheet was cooled at the cooling rate shown in Table 2, wound at the winding temperature shown in Table 2, coiled, and then cooled to room temperature at the cooling rate shown in Table 2.

The obtained hot-rolled sheet was subjected to structure observation, surface quality test, tensile test, impact test, CTOD test, and DWTT test. The test method is as follows.
(1) Structure observation From the obtained hot-rolled sheet, a structure observation specimen was collected, the section perpendicular to the rolling direction (C section) was polished and corroded, and the structure was observed using a scanning electron microscope. .
(2) Surface quality test About the obtained hot-rolled sheet, the surface was observed visually or with a magnifying glass over the whole area of the steel sheet, the presence or absence of cracks was investigated, and the surface quality was evaluated. The case where a surface defect such as a crack having a depth of 100 μm or more occurred was evaluated as x, and the case where it did not occur was evaluated as ◯.
(3) Tensile test A small round bar tensile test piece was taken from the central part of the thickness of the obtained hot-rolled sheet so that the direction perpendicular to the rolling direction (C direction) was the longitudinal direction, and the tensile test was performed. The tensile strength TS was determined.
(3) Impact test V-notch test specimens were taken from the center of the thickness of the obtained hot-rolled sheet so that the direction perpendicular to the rolling direction (C direction) was the longitudinal direction, and conformed to the provisions of JIS Z 2242 Then, a Charpy impact test was performed, and the absorbed energy (J) at a test temperature of −80 ° C. was obtained. The number of specimens was three, and the arithmetic average of the obtained absorbed energy values was determined to obtain the absorbed energy value vE- ₈₀ (J) of the steel sheet.
(4) CTOD test From the obtained hot-rolled sheet, a CTOD test piece is taken so that the direction perpendicular to the rolling direction (C direction) is the longitudinal direction, and tested in accordance with the provisions of BS 7448: Part1 1991. Temperature: A CTOD test was conducted at −10 ° C., a critical opening displacement amount Δc (mm) at −10 ° C. was determined, and ductile crack propagation characteristics were evaluated.
(5) DWTT test From the obtained hot-rolled sheet, a DWTT test piece is collected so that the direction perpendicular to the rolling direction (C direction) is the longitudinal direction, and the DWTT test is performed in accordance with the provisions of ASTM E436. Then, the DWTT temperature (° C.) (the temperature at which the ductile fracture surface ratio becomes 85%) was determined and the toughness was evaluated.

  The obtained results are shown in Table 3.

本発明例はいずれも、表面割れの発生もなく、表面品質に優れ、かつＣＴＯＤ試験における−10℃での限界開口変位量δcが0.25mm以上であり、延性亀裂伝播特性に優れ、高靭性の高張力熱延鋼板となっている。一方、本発明を外れる比較例は、表面割れが発生しているか、延性亀裂伝播特性が低下して、靭性が低下しているか、あるいは表面割れが発生し、かつ延性亀裂伝播特性が低下し靭性が低下している。

Each of the inventive examples has no surface cracking, excellent surface quality, a critical opening displacement amount δc at −10 ° C. in the CTOD test of 0.25 mm or more, excellent ductile crack propagation characteristics, and high toughness. It is a high-tensile hot-rolled steel sheet. On the other hand, the comparative example that departs from the present invention is that surface cracking has occurred, ductile crack propagation characteristics have been reduced, toughness has been reduced, or surface cracks have occurred, and ductile crack propagation characteristics have been reduced toughness Has fallen.

Claims (8)

% By mass
C: 0.02 to 0.08%, Si: 0.5% or less,
Mn: 0.8 to 1.8%, P: 0.025% or less,
S: 0.005% or less, Al: 0.005-0.10%,
N: 0.005% or less, Nb: 0.03-0.10%,
Ti: 0.005-0.05%
A steel material having a composition comprising the balance Fe and unavoidable impurities, a rough rolling step for rough rolling to form a sheet bar, a finish rolling step for subjecting the sheet bar to finish rolling to form a hot rolled sheet, In the method for producing a hot-rolled steel sheet, which is sequentially subjected to a winding process for winding the hot-rolled sheet, the surface bar is cooled to 50 ° C./s or more after the rough rolling process and before the finish rolling process. Surface quality and ductile crack propagation characteristics characterized by performing accelerated cooling that rapidly cools until reaching a temperature of 930 ° C or less exceeding the Ar ₃ transformation point, and then stopping the accelerated cooling and then performing a finish rolling step The manufacturing method of the hot-rolled steel plate which is excellent in it. % By mass
C: 0.02 to 0.08%, Si: 0.5% or less,
Mn: 0.8 to 1.8%, P: 0.025% or less,
S: 0.005% or less, Al: 0.005-0.10%,
N: 0.005% or less, Nb: 0.03-0.10%,
Ti: 0.005-0.05%
A steel material having a composition comprising the balance Fe and unavoidable impurities, a rough rolling step for rough rolling to form a sheet bar, a finish rolling step for subjecting the sheet bar to finish rolling to form a hot rolled sheet, In the method for producing a hot-rolled steel sheet, which is sequentially subjected to a winding process for winding the hot-rolled sheet, the surface layer portion is 50 ° C / at least on the hot-rolled sheet in the middle of finish rolling between the rolling passes at least once in the finish rolling process. After accelerating cooling that rapidly cools until reaching a temperature of 930 ° C or more exceeding the Ar ₃ transformation point at a cooling rate of s or more, stop the accelerated cooling, and finish rolling to obtain a hot-rolled sheet of a predetermined size and shape A method for producing a hot-rolled steel sheet having excellent surface quality and ductile crack propagation characteristics. % By mass
C: 0.02 to 0.08%, Si: 0.5% or less,
Mn: 0.8 to 1.8%, P: 0.025% or less,
S: 0.005% or less, Al: 0.005-0.10%,
N: 0.005% or less, Nb: 0.03-0.10%,
Ti: 0.005-0.05%
A steel material having a composition comprising the balance Fe and unavoidable impurities, a rough rolling step for rough rolling to form a sheet bar, a finish rolling step for subjecting the sheet bar to finish rolling to form a hot rolled sheet, In the method for producing a hot-rolled steel sheet, which is sequentially subjected to a winding process for winding the hot-rolled sheet, after the rough rolling process and before the finish rolling process, the surface bar is cooled to 50 ° C./s or more on the sheet bar. Accelerated cooling is performed to rapidly cool until reaching a temperature of 930 ° C. or more exceeding the Ar ₃ transformation point, and then the accelerated cooling is stopped, and then the finish rolling step is performed, and rolling is performed at least once in the finish rolling step. Between the passes, hot-rolled sheets in the middle of finish rolling are subjected to accelerated cooling that is rapidly cooled at a cooling rate of 50 ° C./s or more until the temperature reaches a temperature of 930 ° C. or more exceeding the Ar ₃ transformation point. And then finish rolling to obtain the desired dimensions Surface quality and manufacturing method of the hot-rolled steel sheet excellent in ductility crack propagation properties characterized by a hot-rolled sheet. The accelerated cooling is cooling in which the surface layer portion is rapidly cooled at a cooling rate of 50 ° C./s or more until the temperature reaches the Ar ₃ transformation point or less, and the finish rolling in the finish rolling step is performed at a reduction rate of (1.1 X Uniform elongation)% or less (Here, uniform elongation: obtained when heated to 950 ° C., cooled to below the Ar ₃ transformation point, and then reheated to 950 ° C. to conduct a high-temperature tensile test. 4. The method for producing a hot-rolled steel sheet according to claim 3, wherein the uniform elongation value (%) in the stress-strain curve).   In addition to the above composition, Cu is selected from 0.005 to 0.5%, Ni: 0.005 to 0.5%, Cr: 0.005 to 0.5%, Mo: 0.005 to 0.3%, and V: 0.005 to 0.3%. The method for producing a hot-rolled steel sheet according to any one of claims 1 to 4, wherein the composition contains one or more kinds.   The winding temperature of the hot-rolled sheet in the winding step is 350 to 700 ° C, and the cooling rate after winding is 5 to 20 ° C / h at the coil central portion. The manufacturing method of the hot-rolled steel plate in any one of. % By mass
C: 0.02 to 0.08%, Si: 0.5% or less,
Mn: 0.8 to 1.8%, P: 0.025% or less,
S: 0.005% or less, Al: 0.005-0.10%,
N: 0.005% or less, Nb: 0.03-0.10%,
Ti: 0.005-0.05%
The surface quality and ductile crack propagation characteristics characterized by having a composition consisting of the balance Fe and inevitable impurities and a structure consisting of a single phase of bainitic ferrite, and a tensile strength TS of 490 MPa or more. Excellent hot-rolled steel sheet.   In addition to the above composition, Cu is selected from 0.005 to 0.5%, Ni: 0.005 to 0.5%, Cr: 0.005 to 0.5%, Mo: 0.005 to 0.3%, and V: 0.005 to 0.3%. The hot-rolled steel sheet according to claim 7, wherein the hot-rolled steel sheet has a composition containing one or more kinds.